1. Field of the Invention
This invention relates to the field of superheterodyne radio sets having digital frequency display means, and particularly to superheterodyne sets having fixed-tuned intermediate frequency filters.
2. The Prior Art
There are known superheterodyne radio sets in which the incoming frequency or received channel is not indicated on a scale but rather by a digital frequency or channel display. Such a radio receiver having a digital frequency or channel display is described, for example, in the German periodical "Funkschau" 1976, No. 9, pages 334-337. A digital indication is based on counting the oscillations of the local oscillator by means of electronic counters. The oscillations to be counted are counted periodically during a definite period of time under the control of a gate, so that each counting interval is referred to as the "gate time". Since, in superhet receivers, the received frequency or more specifically, the carrier frequency of the received signal, differs from the frequency of the oscillator by the value of the intermediate frequency (IF), the IF must be subtracted (or added). In the circuit of the prior art, this is obtained by pre-programming the counter stages of the electronic counter are pre-programmed, prior to the start of the counting, to an initial value such that, after a number of pulses produced during the gate time and corresponding to the intermediate frequency, the highest count or count "zero" of the counter is obtained. Then, the following pulses of the oscillator frequency, counted still within the gate time, exactly correspond to the incoming frequency. In this way, due to the pre-programming, the counted oscillator frequency is corrected by the value of the fixedly pre-programmed IF.
The disadvantage thereof is, however, that the fixed value of the pre-programmed IF is not always identical with the actual value of the IF which is determined by the filters used in the IF section. This is the case particularly if ceramic filters are employed, whose use is desirable because of their advantages. That is, as compared to coil filters, ceramic filters are relatively inexpensive. In their manufacture, they are automatically aligned, so that no subsequent trimming is possible. That is why with ceramic filters, in practice, the value of the IF varies from filter to filter. On the other hand, the fixed value for pre-programming the counting stages also is set in advance, prior to assembling the receiver. For this reason, frequently, and as mentioned, differences are found between the pre-programmed fixed value and the value of the ceramic filters. Such discrepancies entail an incorrect indication of the received frequency. If, for example, the value of the IF predetermined by the ceramic filter is greater than the pre-programmed fixed value, a too high incoming frequency is determined and indicated by the electronic counter. It is true that one might conceive of a subsequent re-programming of the already pre-programmed counting stages to a value corresponding to the respective filter. However, experience has indicated that this prociedure is very complicated and relatively expensive.